Conventionally, various light emitting device driving apparatuses for driving a light emitting device such as a light emitting diode (LED) have been developed.
Here, a configuration of a constant current control circuit included in an LED driver IC is illustrated in FIG. 9 as an example of a conventional light emitting device driving apparatus. A conventional constant current control circuit 100 illustrated in FIG. 9 is a circuit for controlling an LED current IL flowing through an LED 110 to be constant. As illustrated in FIG. 9, the constant current control circuit 100 includes an error amplifier 101, a MOS transistor 102, a resistor 103, a switch 104, a switch 105, a switch 106, and an inverter 107.
The connection relations in the constant current control circuit 100 will be described in detail. A cathode of an LED 110 is connected to a drain of the MOS transistor 102 configured with an n-channel metal-oxide-semiconductor field-effect transistor (MOSFET), and one end of the resistor 103 is connected to a source of the MOS transistor 102. The other end of the resistor 103 is connected to an application terminal of a ground potential. A connection point between the MOS transistor 102 and the resistor 103 is connected to an inverting terminal of the error amplifier 101 via the switch 105. A reference voltage Vref is applied to a non-inverting terminal of the error amplifier 101. A gate of the MOS transistor 102 and one end of the switch 106 are connected to an output terminal of the error amplifier 101 via the switch 104. An application terminal of the ground potential is connected to the other end of the switch 106.
Further, a pulse-type pulse width modulation (PWM) signal Spwm is input to the constant current control circuit 100. The switches 104 and 105 are turned on or off depending on a level of the PWM signal Spwm. Further, the switch 106 is turned on or off according to an output level of the inverter 107, which inverts and outputs an input PWM signal Spwm.
During an ON period (period of high level) of the PWM signal Spwm, the switches 104 and 105 are turned on and the switch 106 is turned off. In this state, the error amplifier 101 drives the MOS transistor 102 such that a feedback voltage Vb, which is generated as an LED current IL, is current/voltage-converted by the resistor 103 and matches the reference voltage Vref, thereby controlling the LED current IL to be constant. Meanwhile, during an OFF period (period of low level) of the PWM signal Spwm, the switches 104 and 105 are turned off and the switch 106 is turned on. Accordingly, the MOS transistor 102 is turned off and the LED current IL does not flow.
Thus, by adjusting an ON duty (ratio of ON period to a certain cycle) of the PWM signal Spwm, a period during which the LED current IL flows is adjusted, which makes it possible to perform dimming of the LED 110.
Recently, automotive displays, which are display devices mounted in vehicles, have been supplied. In the automotive displays, it is required to change brightness depending on daytime vehicle driving, nighttime driving, or driving within a tunnel. Specifically, it is required to increase brightness during daytime driving and to decrease brightness during nighttime driving or driving within a tunnel. Further, in particular, brightness should be further adjusted in order to respond to a user's pupil color. For example, the brightness should be decreased for someone with a lighter pupil color (westerner or the like).
Thus, a dimming ratio (=maximum brightness: minimum brightness) of an LED included in a backlight device provided in an automotive display is required to be 20000:1. For example, when the conventional constant current control circuit 100 as described above is applied to drive such an LED, the following problems may arise.
A cycle of the PWM signal Spwm is required to be 5 ms or less to make it difficult for a user to recognize a flicker. For example, when a cycle is 5 ms and a dimming ratio (ratio of ON period to cycle) is 1/10000, an ON period of the PWM signal Spwm is 500 ns. A waveform example of the PWM signal Spwm and the LED current IL in this case is illustrated in the timing chart of FIG. 10. At a timing t10 at which the PWM signal Spwm rises to a high level, the switches 104 and 105 are turned on and a constant current control is started by the constant current control circuit 100. Thereafter, at a timing t11, the LED current IL reaches a set current Iset and is controlled to be constant. Thereafter, at a timing t12 at which the PWM signal Spwm falls to a low level, the switches 104 and 105 are turned off and the switch 106 is turned on, so that the MOS transistor 102 is turned off, and the LED current IL is reduced to zero.
In other words, the LED current IL reaches the set current Iset through a response delay period Td, which is a period from timing t10 to tn. Since the response delay period Td is shorter than the ON period 500 ns of the PWM signal Spwm as in FIG. 10, the LED current IL can reach the set current Iset within the ON period.
In contrast, when a dimming ratio is 1/20000 at the cycle of 5 ms as described above, the ON period of the PWM signal Spwm is reduced to 250 ns. In this case, a timing chart similar to that illustrated in FIG. 10 is illustrated in FIG. 11. In this case, as illustrated in FIG. 11, since the response delay time Td (period from timing t20 to t22) is longer than the ON period, the LED current IL may not reach the set current Iset at a timing t21 at which the PWM signal Spwm falls to a low level and the LED current IL is turned off.
The reason for the response delay time Td as described above will be described using the timing chart of FIG. 12. As illustrated in FIG. 12, at a timing t30 at which the PWM signal Spwm rises, the switches 104 and 105 are turned on, and a voltage Va, which is an output from the error amplifier 101, rises at a certain response speed. At an early stage, since no current flows through the MOS transistor 102, the LED current IL is zero, but, at a timing t31 at which the voltage Va reaches a threshold voltage Val during rising, current flows through the MOS transistor 102, so that the LED current IL starts to flow.
As the LED current IL rises, the feedback voltage Vb rises, and at a timing t33 at which the feedback voltage Vb reaches the reference voltage Vref during an ON period of the PWM signal Spwm (for example, a case of ON period such as the broken line of FIG. 12), the LED current IL reaches the set current Iset. Thereafter, the LED current IL is controlled to be constant at the set current Iset.
Thus, the response delay time Td is generated as the sum of a period Td1 during which the voltage Va rises to reach a voltage at which the LED current IL starts to flow and a period Td2 from when the LED current IL starts to flow until the LED current IL reaches the set current Iset.
For example, as illustrated in FIG. 13, when a response speed of the error amplifier 101 is increased to rapidly increase the voltage Va, the LED current IL is ringing, which is recognized as flickering to a user. Thus, in order to prevent this, as illustrated in FIG. 12, the response speed of the error amplifier 101 is required to be lowered to a degree and a generation of a certain degree of the response delay time Td is a precondition.